1. Field of the Invention
The present invention relates to an automatic focusing device feasible for a video signal generating apparatus capable of generating a video signal representative of an optical image, e.g., a video camera, still camera or similar imaging apparatus.
2. Description of the Background
Today, imaging apparatuses, including video cameras and still cameras, are extensively used which are capable of picking up a desired object in the form of an optical image via a lens and recording it in a magnetic tape, IC (Integrated Circuit) memory card or similar medium. It is a common practice with this kind of apparatus to mount an automatic focusing device which adjusts the position of the lens automatically so as to focus the optical image of an object onto the photosensitive surface of an image pick-up device.
Automatic focusing devices heretofore proposed include one which determines the sharpness of the image of an object and then moves the lens such that contrast, which is representative of the sharpness, becomes maximum. For example, this type of focusing device uses a focus area defined substantially at the center of a picture and calculates a contrast evaluation value out of high frequency components included in a luminance signal and representative of a contrast. Then, the device moves the lens automatically such that the contrast evaluation value becomes maximum. When such a contrast detection type device is mounted on a still camera capable of producing still pictures, it samples the contrast evaluation value at a plurality of (five to six) positions of a lens and then selects the greatest one of the sampled values.
However, the problem with the conventional contrast detection type automatic focusing device is that a signal representative of contrast evaluation values determined at the various positions of the lens contains noise components. As a result, errors are introduced in the contrast evaluation values. The noise components are generally attributable to noise particular to the image pick-up device and extraneous noise. Particularly, when the contrast of an object is too low to provide the device with sufficient contrast information, noise components become predominant and prevent accurate contrast evaluation values from being produced, thereby obstructing accurate automatic focusing. For example, assuming a dark scene where sufficient illumination is not available, then a video signal representative of the scene is amplified by a great amplification with the result that a great amount of noise components are relatively superposed on a luminance signal derived from the video signal. Since the contrast of an object located in such a scene is often low, the automatic focusing accuracy is further aggravated.
Moreover, a still camera for producing still pictures determines a plurality of contrast evaluation values at a plurality of positions of a lens and then selects the greatest one of them by calculation, as stated previously. Therefore, when the contrast evaluation values are inaccurate, it is impossible to estimate the greatest value which is the key to accurate automatic focusing.